Elevator doors are suspended by rollers which ride on tracks. The doors are driven back and forth along the tracks and are connected by an air cord. The air cord forms a loop and is stretched between a pair of pulleys.
Since air cords tend to lose tension during use, it is important to be able to retension them. If the tension is too low, the doors could separate thereby breaking the safety chain of the elevator and causing the elevator car to stop.
To adjust the tension of the air cord, a device shown in FIG. 1 is used. This device has a vertical, rectangular support plate 12 having edges 14, 16 bent at a right angle through which the ends of air cord 18 pass. The air cord forms a loop around pulleys 20, 22. Springs 24, 26 are slipped over the two ends of the air cord. Sleeves 28, 30 are crimped over the ends of the air cord.
The springs 24, 26 are compressed between the edges and the sleeves to act as continuous tensioners. If the air cord stretches, the springs act to absorb the stretching, thus bringing the tension back to its nominal value.
The tension mechanism is attached to the rear surface of the support plate, a surface which is not visible by a mechanic working at a landing. The support plate itself is attached by means of bolts and nuts to the rear surface of a track for suspension of the doors (not shown).
Such an adjustment device has several disadvantages: the springs make noise because they rub on the sleeves and the edges of the support plate; if an exterior force greater than that exerted by the springs is applied to the air cord, it compresses the springs and therefore loosens the air cord, thus allowing separation of the doors; since the springs are short, they can only accommodate a slight lengthening of the air cord, and they therefore quickly become ineffective; and, once the lengthening has become too extensive, it is difficult to fix because the air cord is relatively inaccessible because it is mounted to the rear of the track.